Just watched it for about the 20th time last night since being introduced to it in English class in high school. This is basically filmmaking perfection to me, or at least as close as it comes. Nicholson is masterful, Fletcher is hypnotically aggravating on the mind and of course you get classic debuts from DeVito and Chris Lloyd.
This movie basically doesn't age to me, and I feel that it's because of the humanistic qualities (stubborness, pride, courage, willpower) that director Milos Forman so perfectly captured that keeps it timeless.